1. Field of the Invention
This present invention relates to an electrochemical method and apparatus for the synthesis of ammonia.
2. Background to the Related Art
Ammonia (NH3) is a colorless alkaline gas that is lighter than air and possesses a unique, penetrating odor. Since nitrogen is an essential element to plant growth, the value of nitrogen compounds as an ingredient of mineral fertilizers, was recognized as early as 1840. Until the early 1900""s, the nitrogen source in farm soils was entirely derived from natural sources. Haber and Bosch pioneered the synthesis of ammonia directly from hydrogen and nitrogen on a commercial scale in 1913. Further developments in large-scale ammonia production for fertilizers have made a significant impact on increasing the world""s food supply.
Virtually every nitrogen atom of a nitrogen compound travels from the atmosphere to its destined chemical combination by way of ammonia. Industrial uses of ammonia as a nitrogen source has recently consumed a greater share of the total ammonia production, accounting for 20% of the world output. Up to 80% of the ammonia produced is used for the production of nitrogen-based fertilizers, accounting for about 3% of the world""s energy consumption. In many developing countries, the capability for ammonia synthesis is the first sign of budding industrialization. In the United States last year there was over 19 billion tons of ammonia produced.
Many methods of ammonia synthesis have been investigated. These methods include the catalytic synthesis of ammonia from its elements using large-scale pressures and temperatures, indirect ammonia synthesis using the steam decomposition of nitrogen based compounds, and the formation of ammonia with the aid of electrical discharge. Only recently has the possibility of using electrochemistry for ammonia synthesis been demonstrated. The electrochemical process is operated at atmospheric pressure and 570xc2x0 C., which is a similar temperature to that used in the Haber-Bosch process. The apparatus consists of a non-porous, strontia-ceria-ytterbia (SCY) perovskite ceramic tube closed at one end and then further enclosed in a ceramic tube. Electrodes, made from polycrystalline palladium films, are deposited on the inner and outer walls of the SCY tube.
Ammonia gas is passed through the system, where the amount of decomposition due to heating can be measured. Subsequently, gaseous hydrogen is passed through the quartz tube and over the anode surface, where the hydrogen is converted to protons:
3H2xe2x86x926H++6exe2x88x92xe2x80x83xe2x80x83(1) 
The protons then diffuse through the solid perovskite electrolyte to the cathode surface, where they come in contact with the nitrogen gas and the following reaction takes place:
N2+6H++6exe2x88x92xe2x86x922NH3xe2x80x83xe2x80x83(2) 
However, the efficiency of the reaction is reduced by the high temperatures needed for the reaction to occur.
Therefore, there remains a need for an improved method of producing ammonia. It would be desirable if the improved method could produce ammonia at lower temperatures and lower pressures, while achieving a greater conversion than existing methods. It would be even further desirable if the improved method were compatible with existing process units, such as being able to use the same hydrogen and nitrogen sources as are used in the Haber-Bosch process.
The present invention provides a method for synthesizing ammonia gas, comprising the steps of providing an electrolyte between an anode and a cathode, providing hydrogen gas to the anode, oxidizing negatively charged nitrogen-containing species present in the electrolyte at the anode to form adsorbed nitrogen species, and reacting the hydrogen with the adsorbed nitrogen species to form ammonia. The negatively charged nitrogen-containing species is preferably a nitride ion, such as lithium nitride, or an azide ion, such as sodium azide.
The reaction is preferably carried out at a temperature between 0 and 1000 Celsius, such as a temperature between 25 and 800 Celsius or between 100 and 700 Celsius, or more preferably between 300 and 600 Celsius, although a lower temperature of between 25 and 150 Celsius may be desirable. The method includes applying a voltage between the anode and the cathode, where the voltage is preferably up to 2 Volts, up to 1 Volt, or up to 0.5 Volt. It is also preferred to apply a current density between the anode and the cathode of up to 2 A/cm2, up to 1 A/cm2, or up to 0.5 A/cm2. Furthermore, the reaction is typically carried out at a pressure between 1 and 250 atmospheres, preferably between 1 and 100 atmospheres, more preferably between 1 and 50 atmospheres, even more preferably between 1 and 20 atmospheres, and most preferably up to 5 atmospheres, including atmospheric pressure.
The hydrogen gas preferably has a purity of greater than 70 percent, more preferably greater than 70 percent. The hydrogen gas is preferably provided to the anode by passing the hydrogen gas through a porous anode substrate. Preferably, the hydrogen gas passes from a first face of the porous anode substrate to a parallel opposite face of the porous anode substrate, wherein the parallel opposite face is in contact with the electrolyte.
The porous anode substrate preferably has porosity greater than 40 percent, but may have porosity greater than 90 percent. Optionally, the porous anode substrate has a thin nonporous, hydrogen-permeable metal film or membrane facing the electrolyte to produce adsorbed atomic hydrogen from hydrogen gas passing there through. The metal membrane can be made from a metal selected from palladium, a palladium alloy, iron, tantalum, and combinations thereof. In addition, it is optional to provide a catalyst disposed on a surface of the metal membrane facing the electrolyte, preferably wherein the catalyst is disposed on at least part of the second surface of the porous anode substrate facing the electrolyte. The metal membrane can also be supported by a matrix formed from a material selected from nickel and nickel-containing alloys. Alternatively, the matrix can be formed from electrically conducting inorganic ceramic materials or a material selected from transition metals and transition metal-containing alloys. Preferably, the metal membrane is a composite comprising a non-noble metal, such as iron, tantalum and the lanthanide metals, having palladium or a palladium-containing alloy on each side of the non-noble metal. In operation, the hydrogen gas may be delivered to the metal membrane from a process selected from steam reformation, partial oxidation, autothermal reformation, and plasma reformation. Alternatively, hydrogen gas may be provided to the porous anode substrate by electrolyzing water. In any of these embodiments, the hydrogen gas may be delivered to the porous anode substrate along with a carrier gas.
It is preferred to produce the negatively charged nitrogen-containing species in the electrolyte by reducing nitrogen gas at the cathode. The nitrogen gas may be delivered through a porous cathode substrate. The porous cathode substrate is preferably made from a metal, metal alloy, ceramic or a combination thereof, most preferably made from nickel, a nickel-containing compound, or a nickel alloy. Alternatively, the porous cathode substrate may be selected from metal carbides, metal borides and metal nitrides. A preferred porous cathode substrate has a pore size of about 0.2 microns. The porous cathode substrate may be coated with a porous electrocatalyst, for example an electrocatalyst selected from transition metals, noble metals, and combinations thereof. The nitrogen gas preferably contains less than 1000 ppm moisture, more preferably less than 100 ppm moisture, and most preferably less than 10 ppm moisture. The moisture may be controlled or reduced by passing the nitrogen gas through a water sorbent material before delivery to the porous cathode. The nitrogen gas should also contain less than 0.1 percent oxygen. Preferably the process includes both providing the hydrogen to the anode catalyst, and reducing nitrogen gas at the cathode to produce negatively charged nitrogen-containing species in the electrolyte, wherein the hydrogen gas and the nitrogen gas are provided at gas pressures greater than the pressure of the reaction.
The electrolyte preferably comprises a molten salt electrolyte that supports migration of the negatively charged nitrogen-containing species between the cathode and the anode. A preferred molten salt electrolyte comprises lithium chloride and potassium chloride, most preferably wherein the molten salt has a greater molar concentration of lithium chloride than potassium chloride. An equally preferred molten salt is selected from the alkali metal tetrachloroaluminates. Preferably, the molten salt electrolyte is charged with a nitride compound, an azide compound, or a combination thereof. The preferred nitride compounds are the nitride salts, such a lithium nitride. Furthermore, the molten salt may further comprise one or more metal salts selected from chlorides, iodides, bromides, sulfides, phosphates, carbonates, and mixtures thereof. Where the metal salt is a metal chloride, the metal chloride may comprise rubidium chloride, cesium chloride, ruthenium chloride, iron chloride, or a mixture thereof. The electrolyte may optionally comprise a salt dissolved in an organic solvent. The method should include maintaining an inert atmosphere over the electrolyte.
The present invention also provides an apparatus comprising a porous anode substrate in fluid communication with a source of hydrogen gas, a porous cathode substrate in fluid communication with a source of nitrogen gas, and an electrolyte disposed within a matrix, wherein the matrix is disposed between the porous anode substrate and the porous cathode substrate. Optionally, a catalyst may be disposed on the porous anode substrate and/or the porous cathode substrate facing the electrolyte matrix. Alternatively, a metal membrane may be disposed on the porous anode substrate facing the electrolyte matrix, preferably including an ammonia generating catalyst disposed on a surface of the metal membrane facing the electrolyte. The preferred catalysts capable of generating ammonia comprise a metal selected from iron, ruthenium and combinations thereof. In particular, the catalyst may be a ruthenium catalyst that is activated by cesium and barium and is supported on a graphite bed, or a barium-activated ruthenium on a magnesium oxide support.
Furthermore, the present invention provides an apparatus comprising a plurality of electrolytic cells and a bipolar plate separating each of the plurality of electrolytic cells. Accordingly, each of the plurality of electrolytic cells comprises a porous anode substrate in fluid communication with a source of hydrogen gas, a porous cathode substrate in fluid communication with a source of nitrogen gas, an electrolyte disposed within a matrix placed between the porous anode substrate and the porous cathode substrate, an anodic fluid flow field in electronic communication with the porous anode substrate opposite the matrix, and a cathodic fluid flow field in electronic communication with the porous cathode substrate opposite the matrix. Preferably, the anodic fluid flow field has a first face that is in electronic communication with the porous anode substrate and a second face in electronic communication with a first bipolar plate, and the cathodic fluid flow field has a first face that is in electronic communication with the porous cathode substrate and a second face in electronic communication with a second bipolar plate. The apparatus will typically further comprise hydrogen gas inlet and outlet manifolds for providing the fluid communication between the source of hydrogen gas and each of the porous anode substrates, and nitrogen gas inlet and outlet manifolds for providing the fluid communication between the source of nitrogen gas and each of the porous cathode substrates. The hydrogen gas manifolds and the nitrogen gas manifolds are each selected from either an internal manifold or an external manifold. In a preferred embodiment, anodic cell frames and cathodic cell frames are disposed around the anode flowfields and cathode flowfields, respectively. These cell frames must be able to withstand the high temperatures, high pressures and harsh chemical environment of the molten salts. Accordingly, the cell frames may be made, for example, from graphite for process temperatures up to 500 Celsius, Inconel or Monel.
In one embodiment, the porous anode substrate and the porous cathode substrate are each selected from metal foams, metal grids, sintered metal particles, sintered metal fibers, and combinations thereof. Preferably, two or more of the metal components of the cell are metallurgically bonded together, such as by a process selected from welding, brazing, soldering, sintering, fusion bonding, vacuum bonding, and combinations thereof. For example, the anodic fluid flow field may be metallurgically bonded to the bipolar plate, the cathodic fluid flow field may be metallurgically bonded to the bipolar plate, the anodic fluid flow field may be metallurgically bonded to the porous anode substrate, the cathodic fluid flow field may be metallurgically bonded to the porous cathode substrate, and combinations thereof.